The present invention relates to drill bits for rock drilling tools and, more particularly, to such drill bits that use hard buttons.
In drill bits 1 used for rock drilling of the general type shown in FIG. 5A (illustrating wear patterns on a known rock drilling bit, Part No. 7738-5348-S48 available from Sandvik Mining and Construction Tools AB, Sandviken, Sweden), sliding friction of the buttons 2, usually cemented carbide buttons, against a hole wall creates diametrical wear on the buttons as shown by the illustrated wear patches 3. It is ordinarily desirable to extend the life of the buttons 2 on such drill bits 1.
The inventors suggest that increasing the amount of area of the buttons 2 that will project into contact with the hole wall should be expected to reduce radial contact pressure on the buttons. The inventors caution, however, that the force is not evenly distributed on all buttons, and likely only two buttons of a bit 1 such as is shown in FIG. 5A are in contact with the hole wall at a given time. The graph of FIG. 4A attempts to illustrate how, as a bit 1 is worn down from 50 mm diameter with new buttons to 48 mm diameter, the amount of wear area increases, i.e., the size of the wear patches 3 on the buttons 2 increases. The following equation is believed to approximate the radial pressure on the carbide buttons 2 of the bit 1:
                                          p            r                    ⁡                      (            r            )                          =                              F            r                                A            ⁡                          (              r              )                                                          1        )            where:pr=Radial pressure on carbide (N/mm2)Fr=Radial force on carbide (N)A(r)=Radial projected area of carbide (mm2)
The volume of wear from the buttons is a function of bit diameter, i.e.:Vc=f(r)  2)where:Vc=Carbide wear volume (mm3)r=radius of bit.The total amount of material (e.g., carbide) to be worn down, i.e., the volume of carbide wear, when the bit is worn from one diameter to another highly influences the bit life. Volume is a truly geometrical function depending on the design of the bit, shown in the graph of FIG. 4B, which illustrates how the volume of the buttons 2 worn away as the bit 1 is worn down from 50 mm diameter with new buttons to 48 mm diameter. As the diameter of the bit becomes smaller, the amount of material that must be worn away increases substantially.
Sliding surfaces in contact under pressure creates wear and the bit wear is dependent on the volume available to be worn down and the pressure applied to the worn area. The inventors have recognized that increasing the area in contact and the volume to be worn down at a specific diameter highly influences bit service life. Consequently, the inventors maintain that, to extend bit life, it is desirable that the area of the bit in contact with the surface of the hole being drilled should increase steeply with decreasing diameter, and more volume of material to be worn down should be provided.
According to an aspect of the present invention, a drill bit for rock drilling tools comprises a drill bit head having a front surface comprising a face surface from which a plurality of cutting surfaces are adapted to extend defining a forward-most end of the drill bit head, the face surface having an outer edge, and a gauge surrounding the face surface, the gauge having an inner edge. A transition region extends in a direction of a longitudinal axis of the drill bit between the outer edge of the face surface and the inner edge of the gauge, and an entirety of the face surface from which the cutting surfaces are adapted to extend is non-flat so that a center of the face surface is axially forward of the outer edge of the face surface.
According to another aspect of the present invention, the gauge comprises a first gauge surface defining a first angle with the longitudinal axis over a first portion of a circumference of the gauge and a second gauge surface defining a second angle with the longitudinal axis over a second portion of the circumference of the gauge.